The present invention relates to the dehumidification of air with the aid of a dehumidifying element that comprises a fibre matrix which has been impregnated with waterglass and which also includes a molecular sieve. More specifically, the invention relates to a method of manufacturing a dehumidifying element.
Air dehumidification is conventionally achieved with a type of dehumidifying element that can be produced by joining together flat and corrugated or fluted fibre material having moisture absorbent properties, to form a laminate that is rolled-up into a rotor-form or stacked in the form of a block. Such laminates are shown in FIG. 3. The dehumidifying element can be likened to corrugated board that has been rolled-up to provide a rotor, or corrugated board that has been cut into lengths and stacked in block form. This element includes a structure that has many parallel channels. The corrugations or flutes are normally from 1.5 to 3 mm high and the channels have a length of from 50 to 400 mm. FIG. 4 shows a part-sector of one such rotor.
A rotor is cut from the roll or block of rolled material and ground, and is often provided with a hub, spokes and/or a skirt. The rotor constitutes xe2x80x9cthe heartxe2x80x9d of an air dehumidifier, wherewith the air is driven through the multiple of channels in the rotor by means of a blower. Because the channel walls contain a moisture absorbing substance, the air will be dry after having passed through the rotor. A small heated air flow passes through a smaller sector of the rotor and expels the air present in this sector. Dry air can be produced continually, by continuously rotating the rotor between the sectors with air to be dehumidified and heated air. The expelled moisture is carried away with the heated air flow in a separate channel system. This principle is also shown in FIG. 5.
SE B 460 705 describes a method of producing a dehumidifying element in which paper that contains ceramic fibres is impregnated with a waterglass solution prior to or subsequent to lamination, wherein the paper is heated and dried subsequent to impregnation, to form hydrated waterglass having a water content of 3-20%. The matrix thus formed is then immersed in acid so as to form a silica-hydrogel.
SE B 460 021 also relates to a dehumidifying element that is comprised of a laminate of sheet of ceramic fibres that is fluted on one side and that has a paper thickness of 0.18-0.25 mm, a flute length of 2.5-4.2 mm and a flute height of 1.5-2.3 mm. This laminate is impregnated with an active silicon dioxide-aluminium oxide-aerogel consisting of 97-85% silicon dioxide and 3-15% aluminium oxide. The element is produced by immersing ceramic paper in an aqueous solution of waterglass and then drying the paper. The paper is then immersed in an aqueous solution of aluminium sulphate and then dried again.
SE B 462 671 also describes a method of producing a dehumidifying element in which a laminate comprising a corrugated or fluted sheet and a facing sheet is impregnated with an aqueous solution of waterglass, followed by drying and heating the laminate to a water content of 5-45% and thereafter immersing the laminate in an aqueous metal salt solution, and finally further drying and heating the laminate.
SE B 469 976 teaches a method of producing dehumidifying elements in which paper webs that contain a mouldable material, such as fibre glass or cellulose, are saturated with concentrated waterglass solution and dried to a dry solids content of 45-65% with respect to waterglass, and then corrugated, whereafter the resultant laminate is dried to a dry solids content of about 60-95%.
EP B 0 642 384 relates to a method of treating dehumidifying elements with the aim of influencing the pore size of the silica gel. This is achieved by treating the elements with an acid, base and a stabilising solution that contains salts of zinc, aluminium and phosphate. Waterglass is applied to the paper and the waterglass then dried, also in this case.
All of the above applications disclose an impregnation step followed by a drying step in conjunction with manufacture of dehumidifying elements that consist of a silica gel matrix. Drying stages are energy demanding and thereby lead to high costs. Moreover, it is difficult to apply sufficient quantities of waterglass when the solution has the low concentration required by known techniques, meaning that the dehumidifying elements produced will have a limited silica gel content and therewith a less than optimal capacity. There is thus a need for improved methods that will lower production costs and increase performance and the quality of the end product.
In certain applications of dehumidifying elements, it is necessary for the dehumidified air to be very dry. For example, a dew point of beneath xe2x88x9240xc2x0 C. is required when drying plastic granules for the manufacture of PET-products, while still lower moisture contents are sometimes required in respect of dehumidifying air in chambers where moisture sensitive products are tested.
A rotor that contains silica gel as a hydroscopic substance can not reach these states without unreasonably high energy consumption by the regeneration process. Thus, there is also a need for novel dehumidifying elements that are able to generate still drier air than that which can be achieved with a dehumidifying element based on silica gel.
It has now been found possible to completely eliminate the drying stage after immersion of paper in the waterglass solution while, at the same time, increasing the amount of silica gel matrix applied and to produce finally a dehumidifying element that is able to dry air to a dew point beneath xe2x88x9240xc2x0 C., by means of a method comprising the steps of
a) providing a piece of paper, such as facing paper and/or fluted paper;
b) immersing said paper in a highly concentrated waterglass solution in which a molecular sieve has been suspended, at a temperature within the range of 45-95xc2x0 C., wherein said suspension of highly concentrated waterglass/molecular sieve has a viscosity of at least 350 mPa.s at a temperature of 45xc2x0 C.; and
c) cooling the immersed paper with air having a temperature of at most 45xc2x0 C. and preferably at most 25xc2x0 C.
A moisture adsorbent that enables air to be dried to very low moisture contents is a molecular sieve. The adsorbent is also called molecular screen, mole screen or zeolite. According to the present invention, a molecular sieve is a highly porous crystalline aluminium silicate which contains in its crystalline structure a large number of very small pores of equal pore opening diameters. Moisture is attracted to the surface and the attraction force is high, particularly on the inside of the pores. In order for molecules to fasten within the pores, it must be possible for the molecules to enter through the pore openings, which have a diameter of between 3 and 10 xc3x85 in respect of different molecular sieves. The water molecule is small and is able to enter the pores, whereas larger molecules are unable to enter and instead pass by the sieve.
Molecular sieves based on aluminium silicate attract polar substances, such as water, with greater or lesser power. A molecular sieve of this nature is thus hydrophilic. The attractive force of the sieve diminishes when the aluminium content decreases. The silicate crystal becomes hydrophobic when containing only a small amount of aluminium or no aluminium at all, and can be used for the adsorption of hydrophobic substances, such as organic solvents for instance. Thus, it is necessary in conjunction with the present invention to utilise hydrophilic molecular sieves, for instance sieves that have a high aluminium silicate content.
The vapour pressure across a hydrophilic molecular sieve that has a given moisture content is much lower than, for instance, the vapour pressure across silica gel that has the same moisture content in a range where said moisture content is relatively low. Expressed in another way, a molecular sieve is able to take-up more water than silica gel, when the ambient air has a low relative humidity. This can be illustrated in a so-called adsorption isotherm in which the adsorption of moisture in weight-% with respect to a moisture free adsorbent is plotted against the relative humidity of the air at equilibrium and at a certain given temperature. FIG. 6 shows examples of such curves.
As a result of these properties, a molecular sieve will have an improved moisture pick-up capacity at elevated temperatures, since the increase in temperature lowers the relative humidity of the air to the region in which molecular sieves have a greater capacity. It is also apparent from the isotherm that the adsorption capacity of silica gel is superior to that of molecular sieves when needing to obtain air that has a very low moisture content.
The term xe2x80x9cwaterglassxe2x80x9d as used in this document relates to aqueous solution of sodium silicate (xe2x80x9csoda waterglassxe2x80x9d) or potassium silicate (xe2x80x9cpotash waterglassxe2x80x9d). Soda waterglass and potash waterglass are often designated as (Na2O)m(SiO2)n and (K2O)m(SiO2)n respectively. As is apparent, the mole ratio between the two oxides (n/m) can vary. In the case of the present invention, soda waterglass with n/m within the range of 3.2-3.5 is preferred, and waterglass with n/m from 3.3 to 3.4 is particularly preferred.
The term xe2x80x9chighly concentrated waterglassxe2x80x9d as used in this document refers to waterglass that has a viscosity of at least 350 mPa.s at 45xc2x0 C. The upper viscosity limit is 800 mPa.s at 95xc2x0 C. The viscosity of highly concentrated waterglass at room temperature is so high as to make it extremely difficult to immerse or dip paper in or into the water-glass at this temperature in practice so as to cause the waterglass to wet the paper. Typical concentrated waterglass according to known technology has a viscosity of up to 200 mPa.s at 20xc2x0 C. Highly concentrated waterglass, on the other hand, has a considerably higher viscosity at 20xc2x0 C. and in its least concentrated form can be likened to cold syrup.
The term xe2x80x9cpaperxe2x80x9d as used in this document relates to sheets produced from organic fibres, such as cellulose, or from inorganic fibres, such as ceramic fibres, glass fibres, slag fibres, carbon fibres, and mineral fibres and mixtures thereof. Inorganic fibres are preferred. It is also preferred in accordance with the invention to use glass fibres and/or mineral fibres with an admixture of up to 20% cellulose fibres or synthetic fibres. The paper will typically have a thickness of 0.1-0.3 mm. The flute height of the fluted paper is typically in the range of 1-5 mm and its flute length is typically in the range of 1.5-7 mm. A typical weight of the paper is 20-50 g/m2.
As mentioned above, the invention has as its starting point a method of manufacturing dehumidifying elements which consist of a paper matrix that has been impregnated with waterglass that includes a molecular sieve for later conversion to silica gel. Instead of using concentrated waterglass as a the silica-gel starting material, there is used a highly concentrated waterglass. This latter waterglass is so highly viscous as to make it impossible in practice to impregnate paper with the waterglass at room temperature. However, the highly concentrated waterglass becomes thinly fluid when heated to a temperature of 45-95xc2x0 C. and will function in the manner of concentrated waterglass. The molecular sieve is suspended in this heated superconcentrated waterglass. The paper to be impregnated is thus immersed into the hot waterglass containing said molecular sieve and then cooled with air heated to a temperature of at most 35xc2x0 C., preferably at most 25xc2x0 C. No drying is required. The cooled impregnated paper also has good adhesive properties and can be readily joined.
Highly concentrated waterglass may, for instance, be produced by vaporising part of the water present in concentrated waterglass prior to manufacture.
In conventional processes, the paper impregnated with waterglass is heat dried, wherewith the waterglass becomes thinly fluid and begins to run. The risk of the waterglass becoming thinly fluid is not found in the process according to the invention, since the highly concentrated waterglass solidifies in the cooling stage.
A subsequent so-called gelling stage can be carried out in different ways, depending on the desired end properties of the dehumidifying element.
The waterglass is converted to a silica gel that binds the molecular sieve powder, by dipping or immersing the paper impregnated with waterglass into or in a concentrated aqueous solution of metal salt (examples of suitable metal salts in this respect are disclosed in SE B 462 671). When the salt used is comprised, for instance, of aluminium sulphate, aluminium phosphate, aluminium nitrate or magnesium sulphate, the gel will have relatively good moisture adsorbency in moist air and in air of average humidity and the rotor will obtain a mixture of the properties of the silica gel and the molecular sieve. However, the deep-drying capacity of the molecular sieve will be unused to some extent, since a great deal of the energy in the heated regenerating air is consumed in expelling the more lightly regenerated water that has bee adsorbed by the silica gel.
An alternative method of converting the waterglass into a good molecular sieve carrier is to expose the impregnated paper to a carbon dioxide atmosphere. The carbon dioxide converts the waterglass into a mechanically highly stable gel that has per se a relatively poor water vapour adsorption capacity. The dehumidifying element is given more pronounced molecular sieve properties in this way. The resultant matrix is washed with water after this stage and dried to the finished product.
This process has many advantages. The highly concentrated waterglass gives a mechanically stronger dehumidifying element, since the final gel/molecular sieve coating becomes higher. The dehumidifying capacity of said element becomes higher for the same reason. Energy consumption, and therewith production costs, become lower, because no drying is required after impregnation. A large amount of energy is required to dry wet paper, since it is necessary to use hot air or IR-lamps to this end. Neither need expensive drying equipment be provided. Finally, the properties of the product are improved, since there is no risk of the waterglass/molecular sieve mixture beginning to run after impregnation.